Regulatory T (Treg) cells play an important role in immune tolerance and prevention of autoimmune diseases such as type I diabetes (TID). Since development and homeostasis of Treg cells depend on dendritic cells (DCs), one challenging task we now face is to understand which pathways that control these functions of DCs. The goal of this proposal is to investigate the mechanisms by which the protein kinase A (PKA) pathway regulates the development and homeostasis of Treg cells through PKA-DC axis, and to explore whether we can control autoimmune diseases through modulating PKA activity in DCs. This project stems from our recent preliminary studies showing that: 1) DC-specific ablation of PKA regulatory subunit R1&#945;(termed here as the DC-PKAR1&#945;ko mutation) results in a markedly increase in total number of Treg cells (up to 50% of total CD4+ T cells);2) PKAR1&#945;ko DCs (termed as PKAhi-DCs) produce high levels of IL-2 and IL-10;and 3) Our microarray and biochemical studies reveal that expression of several genes such as expression of vitamin D receptor (VDR) is significantly up-regulated in PKAhi-DCs. Based on these data, we will test the hypothesis that PKA activation in DC promotes thymic Treg development and mature Treg cell homeostasis through the VDR pathway, and modulation of PKA activity in DC may boost the homeostasis of Treg cells and can potentially be used as a tool to treat and prevent autoimmune diseases such as T1D. Our specific aims are: 1. To investigate the cellular and molecular mechanisms by which PKAhi-DCs regulate Treg cell development and homeostasis. 2. To investigate the role of PKAhi-DCs in prevention and treatment of T1D Result from this study will not only bring further insight into the mechanisms by which DCs regulate Treg development and homeostasis but also provide a new avenue to modulate DC function in the treatment and prevention of autoimmune diseases.